Flow cytometers are typically used to identify various types of cells and biological particles. Flow cytometers of the related art have an optically transparent flow cell made of quartz, formed with a flow path through which the cells to be individually identified flow. The flow of cells passing through the flow path is concentrated in the center portion of the flow path by a sheath liquid concentrically surrounding the flow of cells. The center portion of the flow path is illuminated with a laser beam. When a cell passes through an illumination region, light is scattered depending on the size, shape, and refractive index of the cell. To detect a cell specifically dyed with a fluorescent dye by fluorescence, the wavelength of the laser beam is determined in accordance with the type of the fluorescent dye. In this manner, the fluorescence as well as the scattered light for each of the cells is detected by multiple photodetectors based on the wavelength, enabling a diverse analysis of the cell. Technique of flow cytometry is described in Patent literature 1. A flat-plate flow cell is described in Japanese Patent Application Laid-Open (JP-A) No. 2003-302330 (Patent literature 10) and U.S. Pat. No. 7,105,355 (Patent literature 11). A flow cytometer using a disposable flow cell chip is described in Patent No. 4358888 (Patent literature 14).
Existing cell sorting methods will now be described. The method described in U.S. Pat. No. 3,710,933 (Patent literature 1) or U.S. Pat. No. 3,826,364 (Patent literature 2) is a separation method currently adopted in common products. The method includes discharging droplets of a sample liquid from a nozzle for droplet formation into air, and separating the droplets which include the cells to be separated using an electric field. Japanese Patent Application Laid-Open (JP-A) No. 64-3541 (Patent literature 3) discloses a method that includes the steps of flowing a sheath flow at the periphery of a sample liquid flowing through a flow cell, and shifting charged particles from the sample flow to the sheath flow by applying an electric field to the sample liquid for separation and measurement. Japanese Patent Application Laid-Open (JP-A) No. 1-170853 (Patent literature 4) describes a method that includes a step of applying a pressure pulse to a particle flowing through a flow cell, and thus separating the particles into a flow path which is different from a flow path for steady flow in the flow cell. International Publication No. WO98/10267 (Patent literature 5) discloses a technique that includes applying a field to a flow of microparticles, the flow of which had been narrowed by a sheath flow in the flow cell, and shifting the flow of the microparticles for separation. International Publication No. WO2004/101731 (Patent literature 6) discloses a method of using gel electrodes disposed on both sides of a liquid flow path in a flow cell to apply a charge to the cell and then using an electric field to separate the cell. U.S. Pat. No. 6,808,075 (Patent literature 7) discloses a method that includes the steps of applying a pressure pulse by using a bubble valve forming a meniscus perpendicularly with respect to the flow of particles, and shifting the flow for separation. WO2006/076195 (Patent literature 8) discloses a method that includes a step of applying a pressure pulse as in Patent literature 5, but also includes steps of ejecting each droplet including target particle, and collecting them in a container. U.S. Pat. No. 4,756,427 (Patent literature 9) describes a method that includes steps of measuring each particle in a flow of sample liquid narrowed by a sheath flow, and if it is judged that the particle is a target particle, separating the particles using a piezoelectric element to generate a pulse flow to shift them into a different flow path.
The method for separating cells disclosed in Patent literature 12 is a method in which droplets containing cell flow in oil, and a static electric filed is used to apply a force to the droplet containing the target cell. Patent literature 13 discloses a method wherein a flow cell with branched-flow paths for cell sorting is used, and the target cells are introduced into a flow path for cell sorting by an intermittent flow produced using a piezoelectric element.